The present invention relates to a method and a device for measuring process parameters of a material working process.
High energy beams, in particular laser beams, are used in different ways for material working purposes, for example for cutting, boring and welding different types of material and workpiece geometries. What is common to all processes is that prior to the working process the workpiece is melted and/or partially vaporized. Conversion of the material to be worked into different states of aggregation frequently leads to material defects and process changes during material working. In particular processes during which the material passes through all three states of aggregation, namely the solid, liquid and vapourous states of aggregation, are influenced by a plurality of process parameters.
Below the process parameters influencing the weld geometry during laser welding are stated by way of example:
Weld preparation:
for example edge preparation, material pairing, type of joint
Process control:
for example employment of an additional wire, rate of feed, inert and operating gas
Properties of the working means:
for example guiding of the laser beam, optical focusing device, reproducibility of weld track
Process parameters of the laser means:
for example laser performance, stability of laser performance, beam quality
Further, the development in the automobile sector necessitates more and more complex working geometries which require guidance of the high energy radiation.
To guarantee reproducible working quality, on-line monitoring of the working process is required during which the relevant process parameters are measured and used for controlling the material working process.
From DE 197 41 329 C a method and a device for metal working using plasma-induced high energy radiation is known where the area of the vapour capillaries is monitored for example by a CCD camera for the purpose of controlling and monitoring the working process. In this connection it is envisaged to monitor the area of the vapour capillaries at at least two measuring points to obtain different capillary geometry sizes. Specific areas of the picture taken by the CCD camera are evaluated for determining the capillary geometry sizes. Since all picture data must be transmitted to the evaluation means first, measurement at high monitoring frequency is not possible. Further, the known process does not allow the vapour capillary environment to be monitored such that further independent monitoring and control systems are required for optimized process control.
It is the object of the present invention to provide a method and a device of the aforementioned type for simultaneously acquiring a plurality of process parameters at a high monitoring frequency using a single monitoring means.
The present invention advantageously provides an optical sensor having a dynamic range of more than 70 dB. Such a sensor is capable of simultaneously sensing the area of the vapour capillaries, the melting zone surrounding the vapour capillaries and possibly also a border zone surrounding the melting zone, and allows the measuring signals to be evaluated despite the high contrast range of the picture produced. This allows measuring signals from different sections of the picture field to be simultaneously evaluated for the purpose of determining the process parameters. Since only the measuring signals of sections of the picture field are transmitted to the evaluation means a high monitoring frequency of more than 1 kHz is attainable as the amount of measuring signal data used for evaluation purposes is kept small.
In the picture field sensed by the optical sensor preferably different freely selectable picture sections are defined with exclusively the measuring signals of these picture sections being simultaneously evaluated for the purpose of determining different process parameters which are to be monitored.
Free selection of the picture sections allows a maximum of flexibility with regard to configuration of a monitoring system. Monitoring of specific areas of the melting zone is of importance insofar as the molten mass is fed from the hotter region around the vapour capillaries to the more distant end of the melting zone. Here said molten mass cools down and solidifies. Obviously the hot material is transported at irregular intervals, which affects solidification of the molten mass and may lead to discontinuities and defects in the weld. Thus monitoring of the solidification front at the end of the melting zone is essential with regard to detection of defects occurring in connection with material solidification, for example surface roughness, weld convexity, holes, surface pores etc. Detection of such defects would not be possible by monitoring the vapour capillaries alone.
Measuring signals of picture sections showing the area of the melting zone in front of and at the side of the vapour capillaries may be used for detecting defects occuring during weld preparation.
Measuring signals of picture sections showing the area of the melting zone upstream of the working zone, as seen in the working direction, or the border zone upstream of the melting zone may be used for measuring the weld location and for controlling the laser position or workpiece position. A combination of capillary monitoring, weld zone monitoring and weld tracking offer the user the advantage that all monitoring and control functions can be performed with a single monitoring and control system, which also facilitates operation of the control unit.
The penetration depth of the high energy beam can be detemined from a reduced number of pixles of a picture section showing the center of the vapour capillaries.
The measuring signals of a picture section showing the melting zone downstream of the vapour capillaries, as seen in the working direction, and/or a border area downstream of the melting zone, as seen in the working direction, may be used for measuring the surface topography of the workpiece subjected to the working process.
For special monitoring tasks it is possible to provide a filter in addition to the optical sensor in the beam path such that light with predetermined wavelengths can be filtered.
A CMOS camera is preferably used as optical sensor. The CMOS technology allows pixles of the picture field to be evaluated independently of the overall picture whereby the amount of measuring signal data can be advantageously reduced to a minimum. It is not necessary to acquire all measuring signal data first and then to selectively evaluate them. In this way a high monitoring frequency of more than 1 kHz is attainable.
For this purpose preferably an optical sensor having a dynamic range of more than 100 dB is used.
The focal position of the high energy beam can be determined by measuring the change in light intensity in a linear or rectangular picture section which linearly extends through the vapour capillaries and the two neighboring melting zones.